The best way to coat/paint a board is to use a fastdry clear polyurethane, faster drying the better, Spar is not needed. UV protection is not needed. the board is not in the elements 24/7 like boats.
(For color just have the nice gail at the hardware store add some basic pigments to the polyurethane.) Smile and she mite do it for Free!
I find that polyurethane paint is not the same beast as regular CLEAR poly, when you put on multiple coats of paint, the poly paint tends to act like multiple skins that never drys under each skin causing wrinkles, bubbles, and sliding skins, it is made for one application.

If your poly has pigment in it why would you have to paint for color. I do strips and designs with paint (plast-a-cote) on top of 6 or 7 coats of poly.
Fast dry poly, you can do 7 coats in 2 days, with the plast-a-cote paint drys in two hrs and drys smooth and thick.